Many ground-based navigation applications require directional knowledge. Examples of such applications include aiming/targeting techniques (e.g., for mortar/artillery), navigation of autonomous ground vehicles, and surveying techniques. An important piece of information for use in ground-based navigation is knowledge of one's direction relative to the desired path of travel along the surface of the earth.
Conventional techniques for obtaining directional knowledge include using magnetic compasses, the Global Positioning System (GPS), gyroscopes, and radio signals. These techniques all present certain disadvantages. Magnetic compasses require local calibrations to obtain even marginal accuracy. The GPS requires supporting electronics and cannot be used underground. The GPS also requires the user to be moving, and is susceptible to local area jamming. Conventional gyroscopes are typically too large and expensive given accuracy needs.
North-seeking gyroscopes or gyrocompasses have been developed and are traditionally characterized by reference to a freely rotating gyroscope rotor having damped precession about its own axis of rotation which naturally aligns parallel to the earth's axis of rotation and perpendicular to the local centripetal acceleration vector due to the effective torque induced by the horizontal component of the coriolis force. However, because the rate of change of the gyroscope's angular momentum vector equals the applied torque, a gyroscope having a large angular momentum vector influenced by a comparatively small torque will require significant time to align the angular momentum vector with the axis of rotation producing the torque.
Miniature gyroscopes have been developed and fabricated using micro-electro-mechanical systems (MEMS) technology. An exemplary MEMS sensor is disclosed in U.S. Pat. No. 5,349,855 to Bernstein et al. (hereafter the “Bernstein patent”). MEMS gyroscopes have been used in low cost and performance applications especially automobiles. MEMS gyroscopes have also been used for inertial guidance but typically in conjunction with a GPS to offset gyro inaccuracies. MEMS technology typically refers to small mechanical elements micro-machined into a silicon substrate, which may also contain microcircuitry.
An exemplary north-seeking gyroscope is disclosed in U.S. Pat. No. 5,272,922 to Watson (hereafter the “Watson patent”), the disclosure of which is incorporated herein by reference. This gyroscope uses an angular rate sensor system along with a separate drive signal generator for a motor, and a sensor output signal processing circuit. This approach, however, raises the difficulty of synchronizing the rotation of the motor with the signal processing circuit.